The present invention relates generally to electrical circuits and, more particularly, to an all-NPN bipolar junction transistor current mirror circuit for precisely mirroring an input reference current. A current mirror circuit is designed to replicate a known reference current in one branch of a circuit and apply the replicated reference current into another branch of the circuit. Ideally, the known reference current would be the same as the replicated reference current regardless of the number of loads added to the current mirror circuit.
Despite being constructed of relatively old technology, current mirrors are used in a wide variety of industry applications. For example, current mirrors are used in precision DC biasing circuits, precision gain control circuits, integrated circuits, active loads, precise reference current generation in digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), signal paths in current steering circuits, precision charge pumps, and current followers.
While effective at mirroring currents, current mirror circuits are not without their problems. For example, current mirror circuits are subject to the Early effect. The Early effect causes an appreciable difference between the input and output currents resulting from a difference in the input and output voltages.
Additionally, prior art current mirror circuits suffer from a low common-emitter amplification factor, also known as beta error. As with the Early effect, beta error causes a difference between the input and output currents resulting from the common-emitter amplification factor dropping below an operational value.
There are a number of current mirror circuits well-known in the art which attempt to solve the problems described above. However, actual implementations of these prior art current mirror circuits produce less than ideal results. Specifically, the output current produced by prior art circuits still fail to match the input reference current as precisely as desired.